The fire broke out when one vessel was transferring fuel to another, driving the crews from both ships to jump overboard. Sixteen of the 31 crew were Turkish nationals. At least some of the rest were Indian nationals, according to early reports.
At least 14 people sailors died off Russia-annexed Crimea in the Black Sea on Monday after two ships were engulfed in flames.
There were 31 crew on the two vessels, which were gas tankers sailing under the Tanzanian flag.
Sixteen of the 31 were Turkish citizens, Turkey's AK Party spokesman Omer Celik said, based on information from the Turkish Embassy in Moscow. At least some of the rest of the crew were Indian nationals, according to preliminary reports.
Russia's Federal Agency for Sea and River Transport said the fire erupted while fuel was being pumped from one tanker to another.
The blaze spread from one ship to the other, prompting the crews to jump overboard, according to Russian news agencies.
"A search-and-rescue operation is underway," said Alexei Kravchenko, spokesman for Russia's Federal Agency for Sea and River Transport.
Kravchenko said 14 crew members have been found alive and authorities in the Crimean city of Kerch were preparing to receive the victims, he added.
Δύο εμπορικά πλοία φλέγονται στο στενό του #Kerch, μετά από έκρηξη που σημειώθηκε στο ένα από τα δύο. Συνολικά 31 μέλη πληρώματος, κυρίως Ινδοί και Τούρκοι ναυτικοί - Russian Maritime Agency #Russia #Ukraine pic.twitter.com/OhAgNsnn1v— Sotiris Danezis (@SotirisDanezis) January 21, 2019
The accident struck close to the Kerch Strait, which has become a new flashpoint in tensions between Russia and Ukraine.
In November, Russia fired on and seized three Ukrainian navy vessels in the Kerch Strait as they tried to pass from the Black Sea to the Sea of Azov. The strait connects the two seas.
The confrontation was the first open military incident between Kiev and Moscow since 2014, when Russia annexed Crimea and a pro-Russian insurgency erupted in eastern Ukraine.